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Mild Combustion of non-conventional and liquid fuelsMarco Derudi – Dipartimento di Chimica, Materiali e Ingegneria Chimica / CFALab

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Marco Derudi - CFALab

INTRODUCTION

� Combustion processes are essential for power generation, thus the development of a combustion technology able to accomplish improvement of efficiency with reduction of pollutants emissions, as NOx and soot, is a main concern.

� To improve the thermal efficiency of combustion processes, in the ’80 lots of studies were focused on Heat Exchange and Heat Recovery problems.

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THE BIRTH OF MILD COMBUSTION

Excess enthalpy combustion1

Heat-recirculating combustion: the exhausts can be used to preheat the reactants upstream the flame region2

1Lloyed, Weinberg, Nature, 251,19742Weinberg, Combust. Sci. Technol., 121, 1996

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PRELIMINARY ISSUES

Advantages− Thermal efficiency is increased− Reduced fuel consumption− Possibility to burn low-calorific fuels

Drawbacks− Very high peak temperatures− High NOx emissions (thermal-NOx)− Critical design of the burner

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NOx CONTROL STRATEGIES

Flame control- Temperature- Stoichiometry- Species–dilution and scavenging

Post-flame controlPost-flame NOx reduction by- Reburning- Non-catalytic selective reduction- Catalytic selective reduction (SCR)

Choi, Katsuki, Energy Conv. Management, 42, 2001

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LOW-NOx BURNERS

NOx-control strategies by burner design:

- Staging- Swirling

- Recirculation

These techniques effectively control:- Flame core stoichiometry

- Peak flame temperature

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RECIRCULATION vs FLAMMABILITY

Wünning & Wünning, Prog. Energy Combust. Sci., 23, 1997

UFL

FP AITT

%fuel

AutoIgnition

LFL

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DILUTION, PREHEATING, HIGH-MOMENTUM JETS

Derudi et al., 6th HiTACG Symposium, 2005

Mild

Traditionalflame

Milani, Saponaro, La Termotecnica, 1, 2000

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ADVANCED LOW-NOx TECHNOLOGY

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REGENERATIVE BURNERS - 1

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REGENERATIVE BURNERS - 2

12


SELF-RECUPERATIVE BURNERS

recuperator

combustion chamber

FLOX burner, WS GmbH

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ONE TECHNOLOGY, MANY NAMES

• Heat-recirculating combustion

• Preheated air combustion (PAC)• Diluted Combustion

• Noiseless combustion• High temperature air combustion (HiTAC)

• Flameless oxidation• MILD combustion

(Moderate or Intense Low oxygen Dilution)

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WHY FLAMELESS?

Fuel

T=25°C

Fuel

Fuel

T=25°C

internalrecirculation

Natural Gas 48MWth

“TEA” swirl burner(ANSALDO)

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MILD COMBUSTION - DEFINITION

Derudi, Villani, Rota, Proceedings of the Combustion Institute, 31, 2007

MILD COMBUSTIONTinlet > TSI and ∆∆∆∆T < TSI [K]

Cavaliere, de Joannon, Prog. Energy Combust. Sci., 30, 2004

= 1500 K

= 550 K

= 1000 K

TSelf-IgnitionTraditional combustion

∆∆∆∆T decreases byincreasing dilution

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TYPICAL EXPERIMENTAL TRENDS

Krishnamurthy et al., Proceedings of the Combustion Institute, 32, 2009

Szego et al., Combust. Flame, 154, 2008

MILD

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LAB-SCALE MILD BURNER

Lab-scale burner advantages:low-costlow-timehigh flexibility

Laboratory-scale burner is constituted by three main sections:feeding and pre-heatingburnersampling and measurements

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LAB-SCALE BURNER: DILUTION RATIO

Lab-scale burnerIndustrial burner

ev

a f

Mk

M M=

+

Me = recycled exhaust gases stream

Ma= primary air inlet streamMf = fuel inlet stream

(1 )

1 (1 ) (1 )vR SA IA R

kSA FA SA

− ⋅ += ++ + ⋅ +

SA = 2ary/1ary air volumetric ratioIA = inert/1ary air volumetric ratioFA = fuel/air ratioR = recycle factor

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DILUTION RATIO - K v

Dilution ratio induced by the jets as a function of the dimensionless distance from the air nozzle

SN: 80% of air external to the

nozzle

SN: 25% of air external to the

nozzle F

A EE

V m

dAvk

&

∫=ρ

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HYDROGEN ENRICHED FUELS

H2-enriched fuels are interesting:� ↓ greenhouse gases� from solid fuels (e.g. coal/biomasses gasification)� byproducts of industrial processes (e.g. coke oven gas)

BUT… H2 shows properties that make conventional burners unsuited:� alternative technologies (MILD)� need of research (little work on H2 enriched fuels…)

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RESULTS: CH4/H2

OH(flame marker)

1390 K

1270 K

5.0E-4

0

9.3E-5

0

TemperatureCH2O

(ignition marker)

YOH YCH2O

13751363

13681321

12731279

PredictedPredictedT [K]T [K]

MeasuredMeasuredT [K]T [K]

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RESULTS: CH4/H2

300K 2190 K

1270 K 2000 K

1270 K 1440 K

1270 K 1390 K

flame

MILD

300K 2190 K

Temperature predicted with EDC and DRM-19

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MILD COMBUSTION OF H2 ENRICHED FUELS: NOx

Galletti, Parente, Derudi, Rota, Tognotti, Int. J. Hydrogen Energy, 34, 2009

Contribution of different formation routes to the total NO emissions (dry basis) with

CH4/H2 mixture for different dilution ratios, kv

NNH intermediate mechanism:

N2 + H ↔ NNH

NNH + O ↔ NH + NO

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TOWARD A MILD COMBUSTION OF LIQUID FUELS

• Retrofit a lab-scale burner developed for mild combustion of gases in order to use liquid fuels

- Different nozzles configuration- Atomizer for the direct liquid fuel feeding

• Definition of an experimental procedure for tests with liquid fuels (n-octane, n-dodecane,…)

• Evaluate main characteristic parameters for mild combustion of liquids → requirements for real-size burners design

• Preliminary definition of operating maps for pure (n-octane) and practical (kerosene) fuels

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LAB-SCALE MILD BURNER LAYOUT

Exhausted gases outlet

Primary air + inert inlet

Gaseous Fuel inlet

Secondary air inlet

Quartz-wool

insulation

Preheating oven Refractory insulation

Upper oven for heat

maintenance

Thermocouple

Liquid Fuel inlet

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LIQUID FUEL ATOMIZATION

Cooling water inlet Cooling water outlet

Liquid fuel inlet Nitrogen inlet

The liquid fuel is sprayed directly into the burner

A pneumatic atomizer (supported by a nitrogen stream), water-cooled to avoid fuel pyrolysis, was developed to create a short fuel jet

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EXPERIMENTAL PROCEDURE

1ary air

Gaseousfuel2ary

air

GaseousFuel

1ary air2ary air

Kv can be modulated in a wide range

1 2

1 2

( )a f i a

a a f

M M M R MKv

M M M

+ + ⋅ −=

+ +

PreheatingB Increase of the dilutionC Transition to mildD

KV

Mild clean

Mixed zone

Thermal NOx

Flame Combustion

Ave

rage

Fur

nace

Tem

p.

Extinction

Hig

h C

O e

mis

sion

sA

B

C D

Firing with CH4 or C2H6A

The burner cannot be fired directly with a liquid fuel, so the fuel feed is varied once stable mild combustion conditions are achieved with a gaseous fuel (CH4, C2H6).

GaseousFuel

Air

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MIXED ZONE: ROAD TO MILD

Thermal gradients smoothing

0

20

40

60

80

0 2 4 6 8 10 12

Kv

Con

cent

ratio

n [p

pm]

NO

CO

550

600

650

700

750

800

850

900

950

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Kv

T [°

C]

T top

T half

T bottom

AA

BBCC

BB – Transition

AA –Flame Combustion

CC – Clean mild combustion

NO < 30 ppmCO < 50 ppm

AA

BB

CC

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GAS TO LIQUID TRANSITION

1ary air

Liquidfuel1ary air

Gaseousfuel

Liquidfuel

PURE METHANE OR ETHANE

HYBRID FUEL FEED: GAS and LIQUID

PURE LIQUID FUEL

Single nozzle (SN)

Dual nozzle (DN)

Dual nozzle (DN)

1ary air

Gaseousfuel

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n-OCTANE COMBUSTION

02468

101214161820

0 20 40 60 80 100

% n-octane

Con

cent

ratio

n [p

pm]

NOCO

750

800

850

900

950

1000

1050

0 20 40 60 80 100

% n-octane

T [°

C]

T half

T top

Low thermal gradient

Temperature increase due to the different thermal input

NO < 30 ppm and no CO emissions during the transition

Stable Mild clean regime during and after the gas to liquid

fuel transition

KV=8.75, air excess=14%, air preheated @ 950°C

SN DN

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TEMPERATURE PROFILES

KV=8.75, air excess=14%, air preheated @ 1100°C

KV=5.6

KV=2.7

Exhausts exit

Preheated air nozzle

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MILD CLEAN REGION: n-OCTANE

800

850

900

950

1000

1050

0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5

Kv

T [°

C]

1005°C

KV =1.5

840°C

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REFERENCE COMPONENTS IN REAL FUELS

Ranzi, Energy&Fuels, 20, 2006

20%iso-Alkanes

22%n-Alkanes34%

Cyclo-alkanes

3%Indanes Indenes

and Tetralines

5%2-3 ring aromatics

16%Alkylbenzenes

Typical composition of a kerosene

Concawe, Fuel quality, vehicle technology and their interactions, Report 99-52, 1999

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INFLUENCE OF REFERENCE COMPONENTS

0

2

4

6

8

10

12

14

0 20 40 60 80 100

liquid fuel (%)

NO

x (p

pm d

ry)

n-c8

n-c8/i-c8 95/5

n-c8/i-c8 60/40

0

200

400

600

800

1000

1200

1400

0 2 4 6 8 10 12 14 16 18 20 22 24Air excess [%]

CO

[ppm

]NO Cyc-C6

10% Cyc-C6

20% Cyc-C6

Branched hydrocarbons

Cyclic hydrocarbons

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KEROSENE

Kerosene contains not only linear chain and branched molecules, but also alkyl-benzenes and other cyclic compounds.

GC-MS Analysis

Main fraction: C 10 - C13 hydrocarbonsC11

C10

C12

C13

C14

C9 C15

C16C8

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KEROSENE COMBUSTION

0

100

200

300

400

500

600

13 15 17 19 21

Air excess [%]

CO

[ppm

]

CO (1100°C)

CO (950°C)

KV = 9, air excess=20%, air preheated @ 1100°C

No CO emissions for air excess larger than 20%

Lower CO emissions found for lower air preheating temperatures (it is increased the reactants residence time into the combustion chamber)

OPTIMAL AIR EXCESS

CH4 to KEROSENE TRANSITION

NO < 30 ppm and CO < 1 ppmduring the transition

SOX emissions (max 2 ppm)0

5

10

15

20

0 20 40 60 80 100

% kerosene

Con

cent

ratio

n [p

pm]

NOCOSOx

SN DN

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MILD CLEAN REGION: KEROSENE

800

850

900

950

1000

1050

1100

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

Kv

T [

°C]

1060°C

KV =1.7

850°C

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CONCLUSIONS

• Successful retrofit of a mild combustion burner designed for gaseous fuels to operate with liquid fuels.

• These tests evidenced that it is possible to change, also during a run, the fuel feeding strategy without affecting the mild sustainability.

• Separated nozzles can help to realize mild combustion at lower Kv with respect to the SN layout, thus foreseeing the possibility to obtain mild combustion of liquid wastes and/or high reactive fuels (at moderate/high air temperatures and velocities) at low dilution ratios.

Operating parameters for burners design

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DEVELOPMENTS…

Stadler et al., Proceedings of the Combustion Institute, 32, 2009

Coal mild combustion

Biomass mild combustion

Mild combustion of liquid wastes

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…AND FUTURE PERSPECTIVES

OXY-fuel mild combustion

Kim et al., Proceedings of the Combustion Institute, 31, 2007

High-pressure mild combustion (?)

“OXY-coal” mild combustion (?)

mild combustion of non-conventional and liquid fuelswpage.unina.it/anddanna/capri/capri...

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